Line replaceable units (LRU) are used in commercial and military applications to provide a specific function. A line replaceable unit includes chassis and a plurality of electronic circuits. Some of the electronic components that form the electronic circuits may be sensitive to HIRF. At some level of HIRF intensity a circuit may malfunction causing the LRU to malfunction. A typical LRU has EMI protection such as EMI filter pins in the LRU connectors used to connect the LRU to external cabling and careful shielding of the chassis covers. These protection elements, however, can fail, resulting in the electronic components being subject to the HIRF.
LRUs are tested after assembly to verify that their operation meets specification in an Acceptance Test using factory Test Equipment. In a similar manner, LRUs that have failed and are repaired in the factory or in a test facility are tested to a similar specification using the factory Test Equipment or other test equipment that can accomplish the same testing. These tests are referred to as Continued Airworthiness tests in the case of equipment used on Civil Aircraft.
The testing is conducted on a closed box. That is; the unit is connected to test equipment using cabling similar to that in the vehicle with loads and inputs which simulate normal interfaces.